I have a 01 L300 and just swapped the transmission. It has fresh oil of course and is acting the same way you describe where it's getting stuck in a low gear then heating up and seems like it's slipping as if you were driving a Manual and not fully releasing the clutch while accelerating. The transmission I bought under warranty so wi now have to pull the damn thing out and send it back I think. I will try some Lucas and report back how it works with fresh oil